An exemplary one of conventional ribbon shifting devices will now be described with reference to FIG. 5. A ribbon base 3 is mounted for pivotal motion by means of a support shaft 4 on a carriage 2 which is mounted for reciprocal motion along a platen 1. A stepping motor 5 and gear wheels 6, 7 and 8 connected to be driven in series by the motor 5 are carried on the carriage 2, and a sector gear 9 is secured to the ribbon base 3 and is meshed with the gear wheel 8 so that rotation of the stepping motor 5 may be transmitted via the gear wheels 6, 7 and 8 to the sector gear 9 to pivot the ribbon base 3 in a vertical direction perpendicular to the platen 1 together with a ribbon cassette 10 for a multi-color ribbon thereby to position the ribbon to oppose a portion of the ribbon for a desired color to the platen.
Meanwhile, where the ribbon has up to four color zones, if the width of the ribbon is 19 mm, then the width of each color zone becomes 4.75 mm. The height of a character is normally 3.2 and 4.1 mm, and where it is 4.1 mm, each color zone includes a margin only of 0.325 mm at each of opposite marginal ridges thereof. Accordingly, if the ribbon is mispositioned in a vertical direction, color mixture may occur, that is, a character may be printed partly in a different color in error. In order to prevent such mispositioning, a stopper 11 is mounted on the carriage 2 such that it may abut with the ribbon base 3 to provide a reference position for the ribbon cassette 10 while predetermined pulses are applied to the stepping motor 5 thereby to define a range of movement of the ribbon base 3 to control a shift position of the ribbon relative to the platen.
However, if the ribbon base 3 is abutted with the stopper 11 during movement of the ribbon base 3 to the reference position, the stepping motor 5 will step out since pulses are still being applied to the stepping motor 5. Accordingly, if a predetermined number of pulses are applied to drive the stepping motor 5 after the stepping motor 5 has been stopped, the stepping motor 5 will not be regulated for a difference in phase which has been caused upon such stepping out thereof, and hence color mixture in printing may occur. This problem can be eliminated if a phase of the stepping motor 5 upon stepping out is detected and is fed back to a control circuit to correct pulses to be inputted to the stepping motor 5. However, such means will raise the production cost of the device very much and hence is disadvantageous in practical use.
With such circumstances taken into consideration, normally the stepping motor 5 and the gear wheels 6, 7 and 8 and the sector gear 9 are assembled while the stepping motor 5 is held energized in a predetermined phase with the position of the ribbon base 3 adjusted by the stopper 11. However, such means has a drawback that it will render assembling operations troublesome and increase man-hours, resulting in increase in production costs.